The present invention relates to a focal position detecting optical apparatus which is adapted to detect the displacement between an actual focal position of beams of light focused by an optical system and a desired focal position and which in particular is suitable for use in an automatic focal position adjuster for the optical system of an optical microscope, an optical disc recording and reproducing apparatus or the like.
In recent years, vigorous activity has been directed to developing a so-called optical disc recording and reproducing apparatus in which information, such as video signals, voice signals, electronic computer data or the like, is recorded on and reproduced from a disc at a high density by using light, especially, a laser beam. In such an optical disc recording and reproducing apparatus, laser beam emitted from a light source is required to be focused to a fine beam spot of about 1 .mu.m diameter on the disc by means of an optical system with an objective lens in order for the information to be recorded and reproduced.
Actually, however, it is impossible to obtain a disc surface plane of optical completeness and the disc surface is more or less distorted. In addition, because of eccentricity of the shaft of the turntable carrying the disc, the rotating disc surface vibrates axially with an amplitude of several tens of microns to several hundreds of microns, thus making it impossible to always maintain the correct positional relationship between the objective lens and the disc. As a result, the beam spot projected on the disc greatly changes in its diameter. Accordingly, it is of importance to detect the displacement between a focal point of a light beam focused by means of the objective lens and the disc so as to make it possible to always maintain the correct positional relationship between the objective lens and the disc.
According to a conventional apparatus for detecting the focal position, beams of light focused by a first optical system are projected on an object to be illuminated, a light beam for focal point detection which is derived out of light reflected from or transmitted through the object is received by a photodetector through a second optical system, and changes in density of the focal position detection light beam are detected to determine a displacement between the first optical system and the object. Although the light reflected from the object to be irradiated is herein utilized for the focal point detection light beam, the transmitted light may be utilized for the same from a similar technical standpoint of view.
FIG. 1 illustrates one example of a conventionally known detecting apparatus. In the figure, the first optical system adapted to focus beams of light incident upon the object to be illuminated is not illustrated. This holds true for FIGS. 4, 5, 8, 16 and 17. When the focal point of the first optical system is settled at a desired position 1, a detection light beam as shown by solid line 6 passes through a lens 2 and a portion of the detection light beam which is not shielded by a mask 3 is focused on a gap 5 between photodetectors 4a and 4b, thus forming a focal point image. FIG. 2b illustrates a light beam pattern 7 (focal point image) formed on a plane of the photodetectors 4a and 4b under this condition. When the focal position is displaced as shown at 1' in FIG. 1, the light beam is concentrated on the photodetector 4a as shown by dotted line 6' so that a light beam pattern as shown at 7' in FIG. 2a is formed. A light beam pattern as shown at 7" in FIG. 2c is formed when the focal position is displaced toward the lens 2. Solid line curve 8 in FIG. 3 represents the difference V between outputs of the photodetectors 4a and 4b in relation to displacement .delta. of the focal position and is termed an S curve. The size of the focal point image 7 is of the order of a ratio .lambda./N.A, where N.A. is sin .theta. for an incident angle .theta. of the focused beam 6 shown in FIG. 1 and .lambda. is its wavelength. Taking .lambda.=0.83 .mu.m and N.A=0.1 for instance, the size is then about 10 .mu.m. The detectors 4a and 4b are usually integral and the gap 5 called a dark slit has a width which is made equal to the width of the focal point image 7 obtained by the known apparatus. If the dark slit width is different from the width of the focal point image 7, a region in which irradiations on the photodetectors 4a and 4b are equal occurs when the actual focal point falls forwardly or rearwardly of the desired focal point to provide an S curve as shown by dotted line curve 9 in FIG. 3, thus preventing the detection near the desired focal point. Accordingly, in the conventional apparatus high accuracies are required for designing the detector dark slit width and in setting positions of the detectors. Even with an alternative as shown in FIG. 4 wherein an optical wedge 10 substitutes for the mask 3, as in the first conventional apparatus, requirements of high accuracies are encountered in designing the dark slit width in connection with paired detectors 11 and 12 and setting their positions.
FIG. 5 shows a second example of a known apparatus. When the actual focal point is coincident with a desired focal position 1, a light beam as shown at solid line 16 is refracted by a lens 2 and focused at a tip 13' of a mask 13 to form a focal point image, finally reaching detectors 14a and 14b to form a light beam pattern as shown at 17 in FIG. 6b on surfaces of the detectors 14a and 14b. When the actual focal point is displaced, as shown at 1', from the desired focal position 1 in a direction away from the lens 2, a light beam as shown at dotted line 16' is focused at a site which is closer to the lens 2 than to the mask tip 13', a portion of the light beam directed to the detector 14a is shielded by the mask 13, and only the remaining portion directed to the detector 14b is received thereby. FIG. 6a illustrates a light beam pattern 17' formed on the surfaces of the detectors 14a and 14b under this condition. FIG. 6c depicts a light beam pattern 17" as formed on the detector surfaces when the actual focal point is displaced from the desired focal point toward the lens 2. Solid curve 18 in FIG. 7 is an S curve as obtained with this known apparatus. In this conventional apparatus, constraints on design of the width of a gap 15 (dark slit width) in the integral detectors 14a and 14b and on the setting of their positions are not so critical as in the first known apparatus because this known apparatus is not used normally for detecting the displacement of so a large focal position or length that is provided by a focal point image of the detection light beam formed on the surfaces of the detectors. However, for the desired focal position 1, the focal point image is formed at the tip 13' of the mask 13 to increase the likelihood of light beam scattering, with the result that higher accuracies than those required for the setting of detector positions in the first conventional apparatus are required in order for the inserting position of the mask 13 to be so determined as to cause equal irradiation on the detectors 14a and 14b. Further, since the light beam patterns on the surfaces of the detectors 14a and 14b are inverted when the actual focal point is forwardly or rearwardly of the desired focal point 1, sensitivity for detection of the desired focal point is unnecessarily high as shown by solid line curve 18 in FIG. 7, making it difficult to correct the focal position with the known apparatus. This is because motion of the element to be moved for correction of the focal point tends to pass by a desired position.